The operational efficiency and the overall power output of a gas turbine engine generally increases as the temperature of the hot combustion gas stream increases. High combustion gas stream temperatures, however, may produce higher levels of nitrogen oxides (NOx). Such emissions may be subject to both federal and state regulations in the U.S. and also may be subject to similar regulations abroad. A balancing act thus exists between the benefits of operating the gas turbine engine in an efficient high temperature range while also ensuring that the output of nitrogen oxides and other types of regulated emissions remain well below mandated levels. Moreover, varying load levels, varying ambient conditions, and other types of operational parameters also may have a significant impact on overall gas turbine output, efficiency, and emissions.
Several types of known gas turbine engine designs, such as those using Dry Low NOx (“DLN”) combustors, generally premix the flow of fuel and the flow of air upstream of a reaction or a combustion zone so as to reduce NOx emissions via a number of premixing fuel nozzles. Such premixing tends to reduce peak flame temperatures and, hence, NOx emissions.
Gas turbine engine compressors often include air inlet systems with heating devices for raising the temperature of the incoming airstream. Compressor inlet guide vane icing, surge/stall margin, combustion lean blowout, and the like may result due to cold ambient conditions and/or due to other types of operational parameters. As such, the compressor pressure ratio may be accommodated by bleeding an amount of compressor discharge air and recirculating the air back to the compressor inlet for mixing the cooler ambient air with the bleed portion of the hot compressor discharge air. Such mixing reduces the air density and the mass flow to the gas turbine. Inlet bleed heat also may extend the range of operation of a Dry Low NOx combustion system in an efficient premix mode by keeping the fuel/air ratio as constant as possible.